PCT International Publication WO 2014/016794, whose disclosure is incorporated herein by reference, describes scanning micromirrors based on microelectromechanical systems (MEMS). Embodiments described in this application provide scanning mirror assemblies that include a support structure; a base (also referred to as a gimbal), which is mounted to rotate about a first axis relative to the support structure; and a mirror, which is mounted to rotate about a second axis relative to the base.
In one of the embodiments described in this PCT application, capacitive sensing is used to monitor the rotation of the mirror, using plates of a capacitive sensor that are positioned in proximity to the mirror on opposite sides of the axis of rotation. (In the disclosed embodiment, the plates are angled relative to the plane of the support structure, although in other implementations, the plates may be parallel to the plane of the support structure.) Changes in the capacitance between the plates and the mirror are measured so as to monitor rotation of the mirror.
Strain-based sensors (typically piezoresistive or metallic) are commonly used for measuring deflection of membranes, sensing pressure, measuring deflection of cantilever beams in accelerometers and gyros, and other applications. Various types of strain sensors are known in the art. For example, in a metallic strain sensor, the resistance of a metal resistor changes due to shape deformation that can occur in response to strain. Doped silicon exhibits a strong piezoresistive response, meaning that its resistance changes markedly as a function of the strain itself, which can result from various modes of applied stress. Resistive strain sensing and its use in MEMS are described, for example by Liu in Foundations of MEMS (Illinois ECE Series, Northwestern University, 2005), Chapter 6, which is incorporated herein by reference.